1. Field of the Invention
The present invention relates to the treatment of wastewater and more particularly, but not by way of limitation, to a device and method which uses air bubbles and treatment chemical to remove pollutants therefrom so that the treated water can be discharged or reused.
2. Brief Description of the Prior Art
Federal, state and local regulations limit the amount of hydrocarbons, suspended solids and certain dissolved contaminants which can be discharged to a sewer. Water to be recirculated, e.g., industrial process water, must be cleaned up before reuse. Industrial plants can either clean the wastewater prior to discharge or pay substantial disposal costs for the removal and subsequent treatment of the wastewater. Because liability for spills of wastewater removed for treatment and disposal remains on the generator, it is desirable to treat the water on-site for reuse (also referred to herein as recycle) or discharge.
Suspended solids refers to particulate matter not soluble in water. The name implies that the density of the solids is not great so the particles do not settle. Suspended solids are of two basic types: primary particles (i.e., particles not easily broken up in a wastewater treatment system) and aggregates (small groups of primary particles that can be made to associate into large, tuft-like groups). Aggregates, which settle more rapidly, look like "flocs" of cloth; hence the term "flocculation" is frequently used to refer to the process by which the large aggregates are formed.
When aggregates of oil come together to form oil droplets, the process is called "coalescence." Since oil droplets are liquid they can flow together to form a larger drop--a desirable step, especially if the oil is to be recovered. However, oil droplets can also behave similarly to solid particles, i.e., they can be aggregated or "flocced" in the same way as suspended solids, with each oil droplet retaining its original size and shape. The aggregation of oil particles is an adequate way to remove oil from water, since flocculation is quick. Furthermore, the process brings solids along with the oil to produce wastewater having a reduced level of contaminants.
The speed at which particles settle through water is governed by Stokes' equation which is as follows: ##EQU1## where g=gravity constant
p.sub.2 =particle density PA1 p.sub.1 =liquid density PA1 D=particle diameter PA1 n=liquid viscosity
When the density difference is a negative number, i.e., when the particle density is less than the liquid density, the velocity term V.sub.s will be negative. A negative velocity term means the particle will float rather than settle. A close inspection of Stokes' equation reveals that denser particles settle faster, less dense particles float faster (the absolute value of the negative velocity term will be greater), high viscosity fluids reduce the settling velocity and large particles settle more rapidly than small particles.
Numerous processes and apparatuses have been used to remove contaminants, such as hydrocarbons and suspended solids, from wastewater so the wastewater can be safely discharged or recycled for further use prior to discharge. The most commonly used processes employ gravity separation and air flotation. In gravity separation, clarifiers are used to effect the separation of suspended solids from the wastewater, while API Separators are used to remove hydrocarbons from wastewater; whereas, in air flotation, dissolved air flotation and induced air flotation technoligies are commonly used for the simultaneous removal of hydrocarbons and suspended solids.
Clarification is a gravity separation process wherein the wastewater is passed through a basin or tank and the suspended solids are permitted to settle to the bottom for collection and disposal. Treated wastewater having a reduced amount of suspended solids is removed at a point near the surface, typically over a weir. A skimming device is frequently employed to remove small amounts of floating material. The clarification process assumes the suspended solids have a density greater than that of the wastewater being processed.
Hydrocarbon contaminants, also referred to herein as oil and grease, generally are characterized by densities less than or near the density of water. In an API Separator, the wastewater is passed through a tank wherein the hydrocarbon contaminants tend to rise to the surface. The American Petroleum Institute has calculated the physical dimensions of a tank which will theoretically result in the capture of all hydrocarbons having a particle diameter in excess of 150 microns for a given specific gravity of oil and the specific gravity of water at treatment temperature; hence, the designation API Separator.
In the dissolved air flotation (DAF) process, the wastewater (or a portion thereof) is saturated with air under pressure (typically about 60 psig) and then introduced through a submerged inlet into an open treatment reservoir. At ambient pressure, the solubility of air in water is diminished so that an excess of air is released from the wastewater in the form of very small bubbles. As the air bubbles move toward the surface of the wastewater, they contact and attach to particulate contaminates to form a particle-air aggregate having a lower density than the density of the particle alone. Since the vertical velocity is directly proportional to the difference between the density of water and the density of the particle, a decrease in the density of the particle (because it is now attached to air) results in an increase in the vertical velocity of the particle in water. That is, the particle-air aggregate moves toward the surface of the water more rapidly. The contaminants are skimmed from the surface of the wastewater to produce a treated wastewater containing a reduced amount of contaminants. Typically, a recycle stream of treated wastewater (amounting to about thirty percent of the total flow) is saturated with air in a pressure tank, then combined with the "dirty" wastewater at reduced pressure to distribute air bubbles throughout the recycle/dirty wastewater mixture.
In the induced air flotation (IAF) process, air is induced into the wastewater by mechanical means. The induced air forms bubbles which rise toward the surface of the wastewater. As the bubbles move upward, they contact and become attached to particulate contaminants to promote flotation of the contaminants which are then skimmed (or decanted) from the surface. As in the DAF process, the bubbles function to increase the vertical velocity of the particulate contaminants in accordance with Stokes' Law. The air bubbles produced in the IAF process tend to be substantially larger than those produced in the DAF process, with a resulting smaller surface area for aggregation with particulate contaminants. Unlike the DAF process, however, the IAF process requires no recycle stream or pressure tank. In the typical installation, wastewater is passed serially through several IAF "cells" to produce treated water of acceptable quality for discharge or reuse. While the DAF process has been successful in treating industrial wastewaters having contaminant levels of up to 20,000 milligrams per liter (mg/L), the IAF process is generally considered to be limited to a substantially lower maximum level of contaminants.
The efficiency of the treatment for a given wastewater is dependent, in part, on the size and amount of the air bubbles which attach to the particulate contaminants. Neither the optimum size nor the optimum amount of air bubbles is universal; rather, the optimum amount and size of air bubbles vary with the characteristics of the contaminants in the wastewater. Both the DAF and IAF processes have relatively limited ability to vary the size and amount of air bubbles in a given installation. In the DAF process, the amount of air may be increased by increasing air pressure in the saturation tank or by increasing the percentage of recycled (pressurized) water. In the IAF process, the amount of air is controlled by the design and speed of the mechanical impeller which creates a vortex to induce air into the wastewater.
The DAF and IAF processes encounter limited success where the wastewater contains a high level of contaminants, e.g., suspended solids in excess of 20,000 mg/L for DAF and a somewhat lower limit for IAF. DAF and IAF processes are continuous by design and do not readily lend themselves to batch treatment of wastewater. Further, both the DAF and IAF processes rely on the random contact of rising air bubbles with particulate contaminates to form an air-particle aggregate of reduced density.
The use of chemical treatment to assist in removing pollutants from industrial wastewater is also well known. Chemicals are commonly used for pH adjustment and for reaction with specific pollutants. Furthermore, certain treatment chemicals promote the formation of aggregates of either suspended solids or oil and grease having larger diameters and, therefore, an increased settling velocity. According to Stokes' equation, the vertical velocity of a particle in water is directly proportional to the square of the particle's diameter. Thus, a twofold increase of the particle diameter will produce a fourfold increase in the vertical velocity. A particular contaminant having a diameter 2 D will move either to the surface or to the bottom of a stream of water in one-fourth the time required for a particular contaminant otherwise identical having a diameter D.
In clarification and flotation processes, the use of chemicals improves efficiency by promoting the aggregation of smaller particles into larger particles which settle out (or rise to the surface) more rapidly. The result is an effluent wastewater having a reduced level of suspended solids and hydrocarbons.
While the before mentioned processes have met with limited success in the treatment of wastewater, especially industrial wastewater, new and improved processes and apparatuses are constantly being sought which overcome the problems inherent in the prior art processes and which permit one to comply with the various federal, state and local regulations relating to the discharge of industrial wastewater. It is to such a process and apparatus that the present invention is directed.